1. Field of the Invention
The present invention relates to a method of charging, and more specifically relates to a method of charging for a charging device that charges an electrophotographic charged member built in photocopiers, printers and the other image-forming apparatuses employing the electrophotographic process.
2. Description of the Related Art
In image forming apparatuses using so called electrophotographic process (Carlson process), corona charging devices that utilizes the corona discharge phenomenon have been used as typical means for charging an electrophotographic photoconductor at a desired potential. This method, however, requires a high voltage for causing discharge, which in turn would give electric noises to various peripheral apparatuses. Alternatively, a large quantity of ozone gas that will be generated in discharging would give an unpleasant feeling to people around the machine. To deal with these problems, as alternatives to corona discharging devices, a method has been proposed in which a photoconductor is charged by applying a voltage between the photoconductor and a roller made of conductive resin or a conductive fabric. Nevertheless, this method suffers from another problem. That is, in use of a conductive resin roller, if a micro-area of a photoreceptive layer on the photoconductor to be charged was peeled off to partially expose a conductive substrate such as aluminum, etc., electric current from the roller would converge into the exposed portion, to thereby cause striped charging unevenness extending across the photoconductor in its axial direction.
Specifically, the alternative to the corona charging devices, there has been proposed an electrophotographic contact charging method in which, as shown in FIG. 1, a voltage is applied between an image bearing medium, i.e., photoconductor 1 and a resin roller 65 as contacting member, made of a conductive elastic material, so as to charge photoconductor 1, by bringing resin roller 65 into contact with photoconductor 1.
Another method has been disclosed in, for example, Japanese Patent Application Laid-Open Sho 55 No. 29,837, etc.
FIG. 2 is a perspective view showing an example of the electrophotographic charging device. Here, reference numeral 1 designates a charged member or a photoconductor. The charging device has a charging member which is planted with conductive fabric 75a as contact element to a conductive substrate 75b made of aluminum, etc., and to which a voltage is applied by an unillustrated power supply. Charging of photoconductor 1 is performed by bringing the voltage-applied conductive fabric 75a into contact with photoconductor 1 while the photoconductor to be charged is being rotated.
This charging operation must be performed at the first stage of the image forming process. After having been charged, photoconductor 1 is exposed to light in accordance with image information, bears toner and then transfers the toner-developed image to a transfer material. The toner powder left on photoconductor 1 without having been transferred is removed from photoconductor 1 in a cleaning portion after the transferring step, thus, a series of the image forming procedures is complete.
In spite of the cleaning operation of photoconductor 1 by the cleaning unit, some toner particles can not be removed and may be left on photoconductor i since toner particles are scattered inside the image forming apparatus after a long use. In such cases, the toner particles unremoved are nipped between the contact element and the image supporting medium during the charging operation. The occupation by the toner particles unremoved would inhibit contact between the contact element and the image supporting medium, thus giving rise to a problem that the image supporting medium may not be charged uniformly.
The types of charging devices that use conductive fabric can be generally divided into two classes. That is, fabric is planted like a band in one class, whereas fabric is planted in a roller shape in the other. In either case, the striped charging unevenness can be eliminated which would occur if the conductive resin roller was used. Nevertheless, when a d.c. voltage is applied to the charging member, in other word when a d.c. electric field is generated between the charging member and the photoconductor, no stable charging performance can be obtained because the photoconductor tends to be charged at a higher potential when the system is placed in an high temperature, high humidity environment as compared to when it is in a normal temperature, normal humidity environment. Further, the charging potential in the charger tends to gradually decrease from the start of use, and the variation with the passage of time is too large to bring the device into practical use.
To eliminate the problem caused in the case where only d.c. voltage is applied, a method has been proposed in which an a.c. voltage is superposed or combined to the d.c. voltage.
In disclosures "A Brush Charging and Transferring Device" and "A Brush Charging Device" respectively published in Japanese Patent Application Laid-Open Sho 60 No. 216,361 and Sho 60 No. 220,587, a charging method is described in which a charging member abutted sharing a contact area with a charged member is applied by a combined voltage of d.c. voltage and a.c. voltage.
In Japanese Patent Application Laid-Open Sho 60 No. 216,361, a member made of conductive fabric is used as both the charging member and the transferring member, and the voltages to be combined are defined by the requirements of transfer efficiency and charging uniformity. Specifically, the transfer efficiency limits a combine voltage to fall within a range of 200 to 2 kV. Therefore, when a high d.c. voltage, for example, 1500 V is applied, the a.c. voltage should be limited as low as 200 to 500 V by the requirement of the transfer efficiency and the charging uniformity.
In Japanese Patent Application Laid-Open Sho 60 No. 220,587, the a.c. voltage is specified as low as 300 VRMS, and the amplitude of the a.c. voltage should be 20% or more of the magnitude of the d.c. voltage. Therefore, the d.c. voltage has influence as high as 2,000 V, which is far higher than the desired surface potential. Besides, the frequency of a.c. voltage to be superposed is limited to 500 Hz or more, and the superposition of the a.c. voltage is intended to eliminate the charging failure (striped charging unevenness) caused by regions at which no fabric exists in the charger of the conductive fabric.
Japanese Patent Application Laid-Open Sho 58 No. 40,566 discloses a proposal in which a conductive fabric is formed into a roll-shaped member to be rotated as a charging member, and rotational direction and velocity of the roller are selected.
This disclosure describes that, when a cylindrical, zinc oxide charged member, used as a charged body, is put in parallel contact (in axial direction) with a band-shaped charger, the surface potential of the zinc oxide charged member lowers under a low temperature, low humidity environment. This lowering of the potential is accompanied by a line-shaped image defect. The above disclosure is to eliminate the lowering of the surface potential and the line-shaped image defect. The problem was attributed to a charging phenomenon of the conductive fabric (described in the right, lower column on the third page in Japanese Patent Application Laid-Open Sho 58 No. 40,566.)
Japanese Patent Application Laid-Open Sho 60 No. 220,587 as well as Japanese Patent Application Laid-Open Sho 60 No. 216,361 discloses a method of charging in which a charging member made of a conductive fabric is used to charge a charged member by bringing the charging member into contact with the charged member. Neither of the disclosures, however, make any reference to a charging mechanism of the charging method thereof, to say nothing of the cause and measure of voltage variation due to the charging mechanism. In both disclosures, a relatively low a.c. voltage is superposed over a very high d.c. voltage, e.g., 2,000 V, and particularly, in Japanese Patent Application Laid-Open Sho 60 No. 220,587, a frequency of the a.c. voltage is limited to 500 Hz or more.
The types of charging devices that use conductive fabric can be generally divided into two classes. That is, fabric is planted like a band in one class, whereas fabric is planted in a roller shape in the other. In either case, the stripe-shaped charging unevenness can be eliminated which would occur when the conductive resin roller is used. Nevertheless, when a d.c. current is applied to the charging member, in other words when a d.c. electric field is generated between the charging member and the charged member, stable charging characteristics cannot be obtained because the charged member tends to be charged to a higher potential when the charged member is in an high temperature environment with a high humidity as compared to when it is in a normal temperature environment with a normal humidity. Further, the charging potential in the charging member tends to gradually decrease from the start of use, and the variation with the passage of time is too large to bring the device into practical use.
Thus, various proposals have been made, but all of these could not exclude insufficiency of stability of surface potential. Further, if no consideration is given to the frequency of an a.c. voltage applied, ripples due to the applied a.c. voltage may be overlaid on the charged voltage, and this would on occasions cause a new defect, i.e., unevenness on the image.
Now, consider a case in which a charged member (in this case, an electrophotographic photoconductor) will be charged by using any one of charging members composed of conductive fabric or a conductive fiber aggregation. In this case, the charging member and the charged member are placed opposite to each other sharing a contact point and micro-space therebetween while the charging member being applied with a combination of d.c. and a.c. voltages.
FIGS. 3A and 3B are schematic illustrative views showing a charging mechanism when a photoconductor is impressed by a combination of d.c and a.c. voltages using a charging member made of conductive fabric. Of these, FIG. 3A shows an overall configuration and FIG. 3B is an enlarged view partially showing the vicinity of a contact area. In FIGS. 3A and 3B, reference numeral 1 designates a photoconductor as a charged member, and a charger is designated at 5, on which conductive fibers 5A are planted or adhered.
Referring to FIGS. 3A and 3B, in a case where a tip of a fiber 5A to which a voltage is applied is located opposite to an arbitrary point A on photoconductor 1 while keeping a certain distance, if the applied voltage is greater than a discharge starting threshold voltage (Vth) which is determined depending upon characteristics of photoconductor 1 and the gap, discharge is activated to start charging photoconductor 1. The surface potential (Vsp) will continue to rise until a difference between the applied voltage (Vap) and the surface potential (Vsp) becomes equal to the discharge starting threshold voltage (Vth). When this condition is satisfied, the discharge stops. That is, if the dark attenuation of the potential charged on the photoconductor could be neglected, the relation (Vsp)=(Vap)-(Vth) holds. Then, point A being kept at a certain potential charged, passes out from the area in which discharge is allowed, and moves to a position B where point A comes in contact with conductive fiber 5A. The potential difference between conductive fiber 5A and point A on photoconductor 1 at position B must be, of course, (Vth) as apparent from above. This potential difference causes charges to inject (move) from conductive fiber 5A into point A on photoconductor 1 so as to further increase the potential at point A. In one word, the surface potential is supplied by the combination of discharge effect and charge-injection effect.
The amount of charges injected by the contact is determined depending upon the contact resistance at position B, which in turn depends on the condition of the contact surface. If, for instance, the contact surface is in a high humidity environment and holds moisture thereon, the contact resistance lowers sharply so that the amount of charges injected becomes large. As a result, the surface potential will rise. This mechanism is believed to be a main reason why characteristic of surface potential in this charging method is unstable depending upon environment.
As a means to solve the problem, a proposal has been made in, for example, Japanese Patent Application Laid-Open Sho 56 No. 132,356, in which a constant current power supply is used as a power source for application to a charging member. This method, however suffers from the charge-up problem since current continuously flows through the charging member.
Japanese Patent Publication Hei 3 No. 52,058 describes a proposal for the purpose of uniformalizing surface potential in the similar contact charging method using a charging member and a charged member. However, the charging member used here is limited to roller-shaped or pad-shaped members made of rubbers, and no reference is made to members with conductive fibers planted thereon. According to the disclosure, it is described that when the charging member is applied with a d.c. voltage, the charging process starts above a discharge starting threshold voltage that is determined by Paschen's theory. That is, it can apparently be assumed and understood from the description of the proposal that all the charging is effected only by the discharging and no movement of charges at and through the contact point between the charging member and the charged member occurs. Therefore, a relatively high a.c. voltage that is equal to a charging starting voltage and is two times as high as the discharge starting threshold voltage, is applied between the two members, so that the surface potential may be uniformalized (particularly, spot-shaped charging unevenness can be inhibited) by utilizing discharge effect.
Further, there are several proposals connected with the contact charging method using a charging member and a charged member. These proposals include Japanese Patent Application Laid-Open Hei 3 No. 100,674, Japanese Patent Application Laid-Open Hei 3 No. 100,675, Japanese Patent Application Laid-Open Hei 3 No. 101,764 and Japanese Patent Application Laid-Open Hei 3 No. 101,765. All these applications employ similar charging methods as described in Japanese Patent Publication Hei 3 No. 52,058, and are proposed to limit the frequency of the a.c. charging in order to eliminate unevenness occurring in the development.
The limitations of the frequency described in Japanese Patent Application Laid-Open Hei 3 Nos. 100,674 and 100,675 are to reduce vibration noises caused by the application of a.c. voltage and to increase the number of discharge in the posterior discharge region so as to smooth jaggedness in the surface potential and improve uniformity of the surface potential. In these technologies, the frequency is specified to be 1,000 Hz or less in Japanese Patent Application Laid-Open Hei 3 No. 100,674. The specific frequency in Japanese Patent Application Laid-Open Hei 3 No. 100,675 is 1,000 Hz or less and 2,500 Hz or more, and more preferably 10 Hz or less and 10,000 or more. These ranges are quite different from the frequency range that will be specified later in the present invention.
Repeatedly, Japanese Patent Application Laid-Open Hei 3 No. 100,674 uses the same charging method described in Japanese Patent Publication Hei 3 No. 52,058, and is to reduce unevenness on images caused by the charging unevenness due to influence of variation of the power supply, etc., by limiting frequency of the a.c. charging.
Basically, the techniques described above are to increase sufficiently the number of charge-exchanges caused by virtue of discharging effect so as to smooth the jaggedness of the surface potential, to thereby eliminate image-unevenness. On the other hand, in the charging by the charging member of conductive fabric as exemplified above, both the charge-injection and the discharge effect contribute to the charging mechanism. This charging mechanism can also be applied to the charging member of resin material as found in the prior art if conditions are fitted.
In the charging system based on the mechanism, if an a.c. voltage is applied to such a charging system, it can be easily conceived that the a.c. component may be superposed through the charge-injection on the surface potential. Of course, the possibility cannot be completely negated that an a.c. component applied might be superposed through the discharge effect on the surface potential. Any way, the defect must apparently be attributed to the a.c. voltage applied. Actually, on the final image created by the method appear repeated defects in coincidence with the interval calculated from the process speed and the a.c. frequency.
To sum up, there are two cases for generating a surface potential as follows:
a first case is that a surface potential is generated only through discharge effect; and
A second case is that a surface potential is generated through combination of discharge effect and charge-injection effect. In this case, for example, the charging is effected while a charging member made of conductive fabric and a photoconductor share a contact point and micro-space therebetween. In either case, if charging is effected by a single charging member applied by at least one a.c. voltage, periodic image defects appearing on a final image must be attributed to the a.c. voltage applied.
In order to eliminate image defects due to a.c. voltage, another method has been proposed that differs from the methods described above. Specifically, one or more (at least one) separate charging members (which will be called secondary charging members) are disposed between the previously adopted charging member (which will be called a first charging member) and a developing unit, so that ripples in the surface potential caused by the a.c. voltage applied by the first charging member are eliminated by the secondary charging member or members.
Some disclosures have already been proposed which relate to the charging method in which a plurality of charging members are put in contact with a photoconductor to be charged. Namely, they are Japanese Patent Application Laid-Open Sho 56 No. 91,253 Japanese Patent Application Laid-Open Sho 62 No. 143,072 and Japanese Patent Application Laid-Open Hei 4 No. 16,867.
Of these, a problem referred to in Japanese Patent Application Laid-Open Sho 56 No. 91,253 is the occurrence of damages to the photoconductor, which is attributed in the disclosure to the fact that the photoconductor is charged by the charging member abruptly all at once. Accordingly, a main measure against the problem taken by the invention is that an applied d.c. voltage to a first charging member is set up as low as 200 volts, and d.c. voltages are stepped up from the first through a second to a third charging member. A peak-peak value of the a.c. voltage superposed on a d.c. voltage is limited to 20% or less of the d.c. voltage. Specifically, the peak-peak value of the a.c. voltage applied to the first charging member is as low as 200.times.0.2=40 (V) or less. This publication proposes that the final, third charging member should also be superposed with an a.c. voltage.
A Problem referred to in Japanese Patent Application Laid-Open Sho 62 No. 143,072 is the same with that described in Japanese Patent Application Laid-Open Sho 56 No. 91,253. A main measure against the problem taken by the invention is that the greatest electric resistance is allotted to a first charging member, and the resistance values are reduced step by step through a second to a third charging member. By this arrangement, the potential charged to a photoconductor from the first charging member would be regulated at low level like Japanese Patent Application Laid-Open Sho 56 No. 91,253 so as to prevent the damages to the photoconductor.
Correction of ununiformity of the surface potential due to a.c. voltages applied is a target problem to be solved by Japanese Patent Application Laid-Open Hei 4 No. 16,867. The main feature of the invention disclosed in Japanese Patent Application Laid-Open Hei 4 No. 16,867 is that a.c. voltages that differ in phase one another are applied to a first charging member and a secondary member or members, respectively, so as to correct the ununiformity of the surface potential. Also in this proposal, the final, secondary charging member is to be superposed with an a.c. voltage.
The present inventors have further proceeded to carry out experiments intensively using the just mention prior art means in which the second charging member is provided in addition to the first charging member so as to produce a possible correction effect. As the result of the experiments the following fact was confirmed. That is, in a system including a typical organic photoconductor and charging members made of a conductive resin, as a peak-peak value of an a.c. voltage applied to the first charging member increases up to two times as high as the discharge starting threshold voltage, the a.c. voltage component injected into the photoconductor becomes greater. This naturally requires the voltage applied to a second charging member for the correction to be enhanced. To make matters worse, the resultant surface potential cannot be regulated by the d.c. voltage applied to the first charging member, but becomes large in accordance with increment of the peak-peak value of the a.c. voltage.